The Lewis and Clark Trail is a highly-rated, scenic trail in St. Charles County, not far from the Boone Bridge. We can choose the 5.3-mile Clark trail or the longer 8-mile Lewis trail. P
Check out the trail here: http://www.motrekkers.com/2009/09/lewis-clark-trails-weldon-spring.html
Addendum (May 30, 2011)
Gary and I hiked the Clark trail this past weekend. I'd hiked it before but it's been several years. This is a GREAT trail. Well marked, medium difficulty. Looks like it is ordinarily maintained but with the recent storms there were several trees that blocked the trail throughout. These were actually quite welcome obstacles for us (at one I actually used the grok crawl to go under while Gary climbed over). There was a lot of mud to avoid as well, but this may not be an issue a week later.
The trail head is easy to find off 94 south of 40/61. Take exit 10 off 40/61 and go south for 2.6 miles. On the left there is a gravel parking lot visible as you approach with a sign that says “Weldon Springs Hiking Area” or similar Enter the trail and proceed counter-clockwise. If you come to a fork in the trail and there's no indication which way to go just remember to keep counter-clockwise lol.